Lecture on Hybridization and Sigma/Pi Bonding

Overview of Covalent Bonds

• Covalent Bonds: Formed when two or more atoms share electrons.
• Atomic Orbitals: Include s, p, and d orbitals from which electron regions arise.

Sigma (σ) Bonds

• Definition: Formed by the end-to-end overlap of atomic orbitals creating electron density between the nuclei.
• Hydrogen Example: Two s orbitals from hydrogen overlap to form H₂, where electron density is shared between the nuclei.
• Carbon Example: p orbitals overlap to form a sigma bond when oriented properly.
• Characteristics: Stronger than pi bonds, forming a direct attachment between atoms.
• Single Bond (Bond Order 1): Always a sigma bond.

Pi (π) Bonds

• Definition: Formed when p orbitals overlap side-by-side, with electron density above and below the internodal plane.
• Characteristics: Feature a nodal plane with no electron density directly between nuclei.
• Double Bond (Bond Order 2): One sigma and one pi bond.
• Triple Bond (Bond Order 3): One sigma and two pi bonds.

Molecular Structure Impact

• Sigma Bonds: Shield nuclei, leading to stronger interactions.
• Pi Bonds: Create additional bonding above/below nuclei, adding stability but not directly between nuclei.

Hybridization Theory

• Purpose: Explains the discrepancy between atomic orbital orientation and molecular shape.
• Theory: Mixes orbitals to form hybrid orbitals corresponding to observed molecular shapes.
• Hybridization Examples:
  • sp Hybridization: Mixes 1 s and 1 p orbital, related to two electron regions and 180° bond angles.
  • sp2 Hybridization: Mixes 1 s and 2 p orbitals, related to three electron regions and 120° bond angles.
  • sp3 Hybridization: Mixes 1 s and 3 p orbitals, related to four electron regions and 109° bond angles (typical for methane and other similar molecules).

Connection Between Hybridization and Sigma/Pi Bonds

• Hybrid Orbitals: Used for sigma bonds and lone pairs.
• Unhybridized Orbitals: Used for pi bonds.
• Example with Ammonia:
  • Nitrogen has s and p orbitals, forming sp3 hybrid orbitals.
  • Hybrid orbitals create sigma bonds with hydrogen atoms.
  • Lone pairs reside in hybrid orbitals.

Practical Considerations

• Bond Angles:
  • Ammonia Example: Predicted 109° bond angles due to lone pair interactions.
• Question on Acrylonitrile: Evaluate sigma and pi bonds based on hybridization principles.

Summary

• Key Principle: Bonds form when orbitals overlap.
• Hybridization: Essential for understanding molecular geometry and bonding types.